carbon targets such as solar, wind, and nuclear power, at least in the short to medium term. The discovery of vast reser
Pointmaker THE FACTS ABOUT FUGITIVE METHANE Elizabeth A. Muller and Richard A. Muller SUMMARY
Shale gas production and use is transforming the
Methane, the main component of natural gas, has
energy landscape, both increasing the total
a high greenhouse potential, and opponents
amount of energy resources available and
argue that even if one or two percent of the gas
replacing other fossil fuels (especially coal) for
leaks, the advantage of natural gas over coal
electricity generation. Yet there are still many fears
would be negated.
about the increased use of natural gas, and in particular, the use of shale gas. One such fear is
This estimate is incorrect; over a 100 year time
over methane leaks, both at the production site
span, an implausible 12% of the produced natural
and throughout the supply chain.
gas used today would have to leak in order to negate an advantage over coal. The best current
This paper tries to take an objective look at the
estimates for the average leakage across the
maths around methane leakage. How much
whole supply chain are below 3%; even at 3%
leakage would negate the global warming
leakage natural gas would produce less than half
benefits of using natural gas as compared to
the warming of coal averaged over the 100 years
coal? How concerned should we be about such
following emission.
“fugitive” methane?
Half this 100 year average comes from the first 10
Replacing coal-fired electric power plants with
years; three-quarters from the first 20 years; the
ones using natural gas as a fuel can help reduce
warming at 100 years is almost entirely from the
global greenhouse emissions. New high efficiency
(relatively low) CO2 produced from burned
natural gas plants reduce emissions of carbon
methane, not from the leaked methane itself.
dioxide by 63% if they replace a typical 33% efficient US, UK, or European coal plant, for the same electric power generated. If they replace future coal plants (which would have higher efficiency themselves) the advantage is still large, with carbon dioxide reductions of about 50%.
An additional reason to produce electric power from natural gas is that the legacy advantage of natural gas is enormous; after 100 years, only 0.03% of leaked gas remains in the atmosphere, compared to 36% for remnant carbon dioxide.
1
1. INTRODUCTION
leakage of gas into ground water, ‘flaming
Most of the greenhouse emissions of the future
faucets’ as depicted in the movie Gasland, and
are expected to come from the developing
air and water pollution. 1 In most cases, these
world. That inevitably places severe constraints
concerns could be readily controlled through
on the practicality of paths to near-zero emission
tighter (but reasonable) regulation and fines for
carbon targets such as solar, wind, and nuclear
polluters. In general, enforcement of industry
power, at least in the short to medium term.
best practice for all development would be sufficient.
The discovery of vast reserves of shale gas around the world offers a potentially beneficial
But there is a fourth concern which has not yet
transition approach to less (or zero-) carbon-
been properly addressed: the threat from leaked
intensive energy sources. Natural gas, while not
“fugitive” methane. For there is a concern, held
zero emission, offers the possibility of reducing
by many thoughtful people and others, that the
greenhouse emissions by factors of two to three,
danger of fugitive methane is a compelling
both by replacing older highly-emitting sources,
reason to stop all development of shale gas. For
and by substituting for higher emission power
example, a simple number published by Alvarez
plants that would otherwise be built. In the US,
et al.2 has been widely used by policy makers:
the recent reduction of greenhouse emissions
they
has been significantly aided by the replacement
emissions compared to a coal plant, the
of some old inefficient coal facilities by high
maximum leakage is 3.2%. They do accept that
efficiency natural gas plants.
that value is for immediate effect only, and does
say
that
for
equivalent
greenhouse
not take into account the short lifetime of Although, in principle, natural gas offers a very
methane in the atmosphere.
large greenhouse benefit compared to coal, several objections have been raised. These
It is on this question that the dangers of shale
include the concern that natural gas, even
gas is greatly overestimated. This is the issue
though lower in emissions, is still not a zero-
addressed in this paper.
carbon source of energy. Building new plants
2. SOME SIMPLE (BUT INCORRECT) MATHS
that emit carbon dioxide, no matter how low the emissions may be, subverts the long-term goal of moving to near zero-carbon options.
The concerns over fugitive methane is based on the following true but easily misinterpreted facts
Second, since such plants are typically cheaper
about methane, which makes up 87% to 96% of
than near-zero plants, the increased use of
natural gas.
natural
gas
could,
it
is
said,
delay
the
development of zero-carbon alternatives.
Methane, when released to the atmosphere, has Global Warming Potential (GWP) of 86
A third concern is a belief that fracking, the
over a 20 year period. This means that
mining method behind the rise in natural gas
methane is 86 times more potent as a
production, leads to local problems, including
1
2
The authors have examined most of these concerns previously and their findings are discussed in a series of memos available at www.BerkeleyEarth.org/memos, and www.berkeleyearth.org/papers.
2
R. Alvarez et al., “Greater focus needed on methane leakage from natural gas infrastructure”, Proc. National Acad. Sci. vol 109, 6435-6440, (2012) www.pnas.org/cgi/doi/10.1073/pnas.1202407109.
greenhouse gas than CO2, pound for pound,
water pollution, this has been the inspiration for
averaged over the 20 years following the
a strong movement to ban fracking.
emission. As there are even larger reserves of shale gas in
The
Methane leakage has been observed to
China, and if new technology enables them to
range
on
exploit these, there is therefore a concern that
measurements taken of the air above some
natural gas leakage will completely overwhelm
drilling areas.3
the benefits of developing shale gas.
from
combination
6.2%
of
to
11.7%
these
based
sounds
But the maths described above is incorrect, and
devastating. Here is the way the (incorrect) logic
facts
as a result the conclusions are incorrect. The
follows: Because of methane’s high GWP of 86,
bulleted items below are based on well-known
one might estimate that even 1% leakage, added
numbers, given in the IPCC and other accepted
to the CO2 from burning the gas that didn’t leak,
reports. They give the key facts that were
would negate all advantage over coal. 4 A 2%
ignored in this simple but mistaken calculation.
5
leakage would be twice as bad as coal. Since leakage occurs not only during drilling and
When comparing coal to methane for equal electric power, the 20-year global warming
production, but also during distribution and use,
potential of methane compared to carbon
it may prove impossible to reduce average
dioxide is 11, not 86. The GWP of 86 assumes
leakage below 1%. If the leakage averaged 10%, the warming would be nearly nine times worse
equal weights of methane and CO2. But: (a)
than if coal were used.6 And such leakage has
methane is lighter than CO2, molecule per molecule, by a factor
been reported for at least one site.
7
of 0.36; (b) coal
produces only 0.60 of the heat, molecule per Based on such simple (but wrong) estimations,
molecule (since it contains less hydrogen); 8
the case against natural gas appears to be
and (c) for equal heat, coal produces only
overwhelming. Together with worries over local
0.61 as much electric power.
3
4
5
Karion, A., et al. (2013), Methane emissions estimate from airborne measurements over a western United States natural gas field, Geophys. Res. Lett., 40, 4393–4397, doi:10.1002/grl.50811. The following is not meant to be an example of a calculation found in the peer-reviewed literature; rather it is the sort of calculation done informally by knowledgeable people (including one Nobel Laureate) who were concerned and not being careful. For 1% leakage, the effect of fugitive methane is assumed to be 86 x 0.01 = 0.86 compared to coal. Add in 0.31 from carbon dioxide released through burning, and you get 0.86+0.31 = 1.17 times worse than coal. We will show that this commonly-made calculation is incorrect. 86 x 0.02 = 1.72 from methane leakage. Add in 0.31 from carbon dioxide released through burning, and you get 1.72+0.31=2.03, meaning that leaked methane would be 2.03 times worse than carbon dioxide. We
9
Combine
will show that this commonly made calculation is incorrect. 6
86 x 0.1 = 8.60 from methane leakage. Add in 0.28 from carbon dioxide released through burning of the remaining methane, and you get 8.60+0.28=8.88, meaning that leaked methane would be 8.88 times worse than carbon dioxide. We will show that this is a commonly made but incorrect calculation.
7
The molecular weight of CH4 is 16, and of CO2 is 44. The ratio 18/44 = 0.36.
8
Counting by molecules is required since one molecule of leaked methane replaces one molecule of produced CO2. Coal results in 92 grams CO2 per MJ heat, while gas results in 55 grams CO2 per MJ; see Hayhoe, H. Kheshgi, A. Jain, D. Wuebbles, Climate Change vol 54, 107-139 (2002); DOI 10.1023/A:1015737505552.
9
We assume 54% efficiency for new natural gas plants, and 33% for the coal plants they are replacing. Newer
3
these, and the GWP of methane, for equal
electric energy produced, 100 year average).
power, is reduced from 86 to 11. When
In some extensive regions (the Marcellus in
considering substituting a methane plant for
Pennsylvania) recent measurement in the air
an equal power coal plant, 11 is the
above the sites indicate leakage has been
appropriate GWP, not 86. This is not in
kept below 0.41%. 10 The bulk of the leakage
dispute among scientists. In the following
comes from a small number of “super
calculations, however, we will use the
emitters”. The cost to reduce the emissions
traditional value of 86 and keep track of the
from these super emitter sites can be
weight and efficiency factors to keep our
recovered by the added value of the gas. This
maths transparent.
is a case where the environmental motive and the profit motive are aligned, and there is
Legacy warming from fugitive methane is
economic incentive to reduce leakage from
minuscule compared to that of carbon
identified super emitters.
dioxide. The 20-year average typically used in
the
comparisons
the
Ignoring the leakage, when used for electricity
from
generation the benefits of natural gas over coal
atmospheric methane destruction. Nor does
are huge; new plants replacing the average US
the 100-year average, since most of that
coal plants produce only 37% the carbon
average effect comes from the first few
dioxide. That means switching electric power
decades. Only 0.03% of fugitive methane
production to natural gas could extend the time
released today will still be in the atmosphere
available to develop zero-carbon solutions
100 years from now. In contrast, 36% of the
significantly. In fact, some people oppose natural
carbon dioxide will linger. The difference in
gas specifically for this reason, because it
atmospheric lifetime completely overwhelms
reduces the urgency to develop carbon-free
the higher greenhouse effect of methane,
alternatives. Z. Hausfather has analysed this in
making carbon dioxide, not fugitive methane,
some detail and at different leakage rates, and
the long-term threat. The commonly-used
shown that even if such alternatives are delayed
limit of 3.2% leakage totally ignores this
by natural gas use, the benefits in slowing
legacy effect.
greenhouse warming are substantial.11
enormous
Average
leakage
dangerous leakage
doesn’t
subsequent
show
reductions
below
These facts are not controversial. Nevertheless,
levels. Although up to 10%
they surprise many people because they conflict
has
been
today
is
reported,
far the
best
with what they have read or heard in media
estimates for the average leakage today,
summaries. In order to reconcile these facts with
including by the EPA, are under 3%. Yet even
those that are typically discussed by those
with 3% leakage, natural gas would cause less
opposed to natural gas, we’ll go into more detail.
than half the warming of coal (assuming same
power plants can have higher efficiency; the highest in the world may be the Avedøre Power Station in Denmark; it achieves 49% efficiency. 10
4
Quantifying atmospheric methane emissions from the Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions, J. Peischl, T. B. Ryerson, K. C. Aikin, J. A. de Gouw, J. B. Gilman, J. S. Holloway,
B. M. Lerner, R. Nadkarni, J. A. Neuman, J. B. Nowak, M. Trainer, C. Warneke and D. D. Parrish, JGR Atmospheres, DOI: 10.1002/2014JD022697. 11
Z. Hausfather, Climate Impacts of Coal and Natural Gas, Berkeley Earth memo available at http://static.berkeleyearth.org/pdf/climate-impacts-ofcoal-and-natural-gas.pdf.
3. GREENHOUSE POTENCY OF METHANE COMPARED TO CARBON DIOXIDE
electrical energy, natural gas plants produce less
The global warming potential, GWP, of methane is
would be for the same electric energy output.
defined as the greenhouse effect that a kilogram
Much less methane (by weight) is used to get the
of methane will have when released to the
same output as from coal. 12 For the same heat
atmosphere in a single pulse, compared to the
energy produced, burning natural gas produces
effect from the release of a kilogram of carbon
only 60% the carbon dioxide as burning coal. In
dioxide. The IPCC gives the GWP of methane as
addition, as we said earlier, heat from natural gas
86, averaged over the first 20 years after release,
is more efficiently used at turning heat into
and as 34, averaged over the first century. Note
electricity. The average US, UK, or European coal
that these GWPs are averages. In fact, most of the
plant produces electricity with 33% efficiency.
100-year GWP comes from the 30 years, before
Modern combined-cycle natural gas plants have
the methane leaves the atmosphere.
54% efficiency.13 That high efficiency comes from
CO2 than do coal plants, so a better comparison
burning the natural gas directly in a turbine, and There has been quite a bit of discussion over
then making use of the “waste heat” to run a
which
for
second steam turbine; the two-stage system is
comparison. Those who worry that we could reach
called a combined cycle gas plant. The higher
a tipping point in the next 20 years prefer to use
efficiency reduces the relative carbon dioxide
the 20-year horizon. Those who worry about long-
produced even further, from 60% (above) to 37%
term warming, and point out that previously
of the emissions of the coal plant that is being
anticipated tipping points never materialized, may
replaced. Put another way, the emissions from a
prefer the 100-year horizon. Rather than get into
coal plant are 2.7 times greater than those from a
this discussion, we provide numbers for all of the
natural gas plant that produces equal electric
commonly used time horizons (see Table 1 below).
power. We call 2.7 the “advantage factor” of
is
the
most
useful
timeframe
natural gas. Calculations of GWP by both weight
The standard GWP refers to equal weights of
and energy output are provided in Table 1.
methane and carbon dioxide. But for the same
12
13
For methane, the greater energy per molecule comes from the fact that methane contains more hydrogen than coal. The water vapour produced when hydrogen burns quickly condenses and does not contribute to warming.
The efficiency is sometimes stated as 60%, but that is calculated using the lower heating value. For a fair comparison, we use the higher heating value consistently for both coal and natural gas, and that reduces the efficiency from 60% to 54%.
Table 1. GWP of Methane by Weight and by Energy Output (referenced to GWP = 1 for carbon dioxide) 0 yr
0 to 20 yr average
20 yr
0 to 100 yr average
100 yr*
GWP of methane per weight
120
86
34
34
1.5
GWP of methane per energy output
15
11
4.3
4.3
0.5
* After 100 years, the methane from a pulse injection is virtually gone from the atmosphere; the GWP is dominated by the CO2 produced in the atmosphere originating from the chemical reactions that destroyed CH4. A reasonable estimate for that is 1.5 kg of CO2 produced for every kg of CH4 leaked to the atmosphere.i Table 1 includes the CO2 produced in the atmosphere in all timeframes, though it is significant only in the 100-year timeframe. i
N P Myhrvold and K Caldeira, Greenhouse gases, climate change and the transition from coal to low-carbon electricity, (2012). Environ. Res. Lett. 7 014019 doi:10.1088/1748-9326/7/1/014019.
5
Of course, one can still use the IPCC values per
atmosphere, and is removed with a half-life of 8.6
unit weight, but they need to be used with care,
years.14
compensating for different weights required. For fugitive
After 100 years only 0.03% of methane remains in
methane is reduced enormously because of its
the atmosphere. This means that if we were to
short atmospheric lifetime. This will be discussed
implement zero carbon solutions on a global
next.
scale in the future, in order to bring global
longer
durations,
the
potency
of
temperatures back down, it is better to have
4. LEGACY WARMING FROM FUGITIVE METHANE
emitted dioxide.
more
methane,
and
less
carbon
15
Carbon dioxide has a long legacy, and persists in the atmosphere far into the future. After 100 years, 36% of emitted carbon dioxide is still in the air.
Methane,
the
dominant
greenhouse
component of natural gas, is strikingly different.
So from a legacy perspective, carbon dioxide is much worse than methane. Figure 1 shows the persistence of methane and carbon dioxide in the atmosphere.16
Methane reacts with hydroxide radicals in the 14
15
The IPCC gives the “lifetime” as 12.4 years; however, that is not the half-life but the mean life, the time it takes the gas to reduce to 36.8% of its initial value. The half-life is the time that it takes for the gas to reduce to half of its initial value. Mathematically, halflife = ln(2) x mean-life = 0.693 x mean-life. This is discussed in detail in Z. Hausfather, Climate Impacts of Coal and Natural Gas, Berkeley Earth memo available at:
http://static.berkeleyearth.org/pdf/climate-impacts-ofcoal-and-natural-gas.pdf. 16
The data is based on the memo of Z. Hausfather, Climate Impacts of Coal and Natural Gas, Berkeley Earth memo available at: http://static.berkeleyearth.org/pdf/climate-impacts-ofcoal-and-natural-gas.pdf. The fraction of initial CO2 left in the atmosphere as a function of time was calculated using CO2(t) = 0.217 +
Figure 1. The persistence of carbon dioxide and methane in the atmosphere, as a function of time.* The legacy effect of methane (CH4) is miniscule compared to that of carbon dioxide (CO2). 100 90 80 70
percent
60 50 40
CO2
30
ratio
20
CH
4
10 0
0
20
40
60
years since emission
6
80
100
The virtual total disappearance of methane
could be tipping points or other factors that
surprises some people, since the IPCC value for
would
the 100-year global warming potential still has
assumptions
the relatively high value of 34. But that number
previously predicted tipping points have not
refers to the average potential during the first
materialised, the next one possibly could. This is
100 years after the emission. Half of that “100-
true, and possible future tipping points should be
year average” comes from the first decade, and
considered when thinking about longer time
three-quarters comes from the first two decades.
horizons. However, even after a tipping point,
At 20 years, 80% of the methane is already gone;
methane will still have a dramatically shorter
(converted to carbon dioxide); at 100 years,
legacy
99.97% is gone. Of course, if the power plant
materialization of future tipping points are a
continues to operate, there will be new methane
possibility, not a certainty.
dramatically
change
about
than
global
carbon
our
underlying
warming.
dioxide,
While
and
the
added from any ongoing leaks.
5. NATURAL GAS ADVANTAGE Over longer time frames, the lower warming legacy
of
methane
becomes
even
What
more
percent
natural
gas
leakage
would
completely negate its benefit compared to coal?
remarkable. If we dump a million tons of carbon
Because of the lifetime difference in the
dioxide into the atmosphere today, then even
atmosphere, the answer depends on time scale
after one thousand years, 22% would still be in
of interest. Table 2 gives the results over several
the air. On the other hand, if we dump a million
time frames.
tons of methane into the atmosphere, then after one thousand years it will be totally gone. By that
The calculations include the carbon dioxide from
we mean that less than one atom of those million
the methane that burns in addition to the
tons is expected to still be in the atmosphere. If
methane leaked directly into the atmosphere.
the harm to future generations is the salient
We assume that the natural gas is pure methane
issue, then it is critical to note that methane goes
(of all the constituents, only methane has a high
away rapidly while large amounts of carbon
global warming potential, if it is less than 100%
dioxide persist. The difference is dramatic.
methane the global warming advantage of
Some argue that it is wrong to use longer time
natural gas over coal increases). The detailed
horizons when comparing the long-term impacts
calculations for Table 2 are provided in this
of greenhouse gas emissions because there
footnote.17
0.259 e-t/172.9 + 0.338 e-t/18.51 + 0.186 e-t/1.186. The equation was based on the work of N. Myhrvold and K. Caldeira, Envir. Res. Lett. 7 (2012), doi: 10.1088/1748-9327/7/1/014019. 17
send 1 kg of natural gas (taken in the worst case to be pure methane) into the power plant. Then f kg will leak, and (1-f) kg will burn. Each atom that burns combines with oxygen to make CO2. Because the molecular weight of CO2 is 44, and that of methane is 16, the burning produces (44/16)(1-f) = 2.75 (1-f) kg of CO2. In addition to this, the leaked methane will have a CO 2 equivalent
The equation for the leakage fe for equivalence to coal, is fe = 4.6/(GWP + 4.6). This is derived as follows. Let f be the fraction of methane that leaks. We use the global warming potential GWP per unit weight. For simplicity, we
Table 2. Methane Leakage to Lose Global Warming Advantage vs Coal
% leakage for coal equivalence
0 yr
0 to 20 yr average
20 yr
0 to 100 yr average
100 yr*
3.8%
5.3%
12%
12%
65%
* At 100 yr, the warming contribution is dominated by the atmospheric methane that has reacted in the atmosphere to create CO2.13
7
The calculation for methane leakage at the rate
Another way of thinking about the same issue is
of 5.3% shows that at this leakage rate, the effect
to ask how much better is natural gas than coal
of fugitive methane, when added to the carbon
at certain leakage rates, and over certain
dioxide warming effect of the 94.7% of gas that
timeframes. We call this advantage factor A, and
is
greenhouse
an equation for it is derived in footnote 17. With
emissions of coal. It does not exceed it, as one
no leakage, natural gas is 3.2 times better than
may wrongly deduce from the simple statement
coal. For 3% leakage, the 100-year-average
that “methane is 86 times more potent than
advantage drops to 2.3. The natural gas
carbon dioxide”.
advantage for various leakages and time periods
burned,
just
matches
the
Another way of saying this is that if you want to build a natural gas plant instead of a coal plant (and are considering the timeframe of 20 years), 5.3% methane leakage over the entire lifecycle
is shown in the Table 3. This shows that even if you are most concerned with the near-future, natural gas is dramatically better than coal.
of natural gas would put you at greenhouse
6. AVERAGE LEAKAGE TODAY IS FAR BELOW DANGEROUS LEVELS
emissions equivalency with coal. As shown, a
How much natural gas is actually leaking? In 2011,
similar calculation for a 100-year period (average
concern over the potential threat of fugitive
GWP of 34) indicates that the methane leakage
methane was ignited by an article by Robert
would have to be 12% to match the warming
Howarth and collaborators.18 They estimated that
effect of the replaced coal plant. If we were
leakage from new hydraulically fractured natural
concerned about the legacy at 100 years (not at
gas wells and supply chain could be as high as
the average from now until then), then absurd
7.9%. They obtained this number by taking their
amounts of methane would have to leak, 65%, to
highest value for leakage from a conventional
have the greenhouse effect of a coal plant. When
gas well, 6%, and adding on an additional
worrying about impacts on future generations,
leakage of 1.9% that could occur during the
natural gas use today is far superior to coal.
flowback operation (done for shale gas wells but not for conventional natural gas operations).
effect of (GWP)(f), making a total CO2 equivalent global warming effect equal to the sum: 2.75 (1-f) + (GWP)(f). A coal plant, for the same electric power generated produces 2.68 times as much CO2 as does the methane plant, equal to (2.68)(2.75)(1-f) kg of CO2. The “methane advantage” factor A is the ratio of this to that from the methane plant: 7.37 (1 − 𝑓) 𝐴= 2.75 (1 − 𝑓) + 𝐺𝑊𝑃 𝑓
For coal/methane equivalence value fe, we take A = 1 and solve for f yields fe = 4.6/(GWP + 4.6). 18
Howarth, R.W., Santoro, R., Ingraffea, A., 2011. Methane and the greenhouse-gas footprint of natural gas from shale formations. Climatic Change 106, 679–690.
Table 3. Global warming advantage of natural gas vs. coal electrical plants at different leakage rates and over different time horizons
8
0 yr
0 to 20 yr average
20 yr
0 to 100 yr average
100 yr
At 3% leakage
1.4
1.6
2.3
2.3
3.2
At 2% leakage
1.7
1.9
2.6
2.5
3.2
At 1% leakage
2.2
2.4
2.8
2.8
3.2
At 0% leakage
3.2
3.2
3.2
3.2
3.2
Such leakage happens if the flowback methane
We now know that Howarth’s leakage value of
is vented to the atmosphere rather than flared.
7.9% was high; a better estimate is 1.9% to 2.6%.
They were being cautionary; in their data from 5
A detailed review of leakage studies was
wells, only one had substantial (1.3%) methane
published in 2014 by Brandt et al.19 and further
emission during flowback.
analysed and summarised by Hausfather.20 The official leakage rates from well inventories report
A more reasonable reading of Howarth would not
leakages averaging 1.5%; other studies show
include the very high potential emissions from
higher levels of 2% to 4%, including some “super
transport, storage and distribution, which added
emitters” that leak 6% to 10%. Brandt concludes
3.6% to the upper range. That leaves the total at
that the average emissions were probably
4.3%, more consistent with other estimates.
between 1.9% and 2.6%.21 A recent report of the
Howarth’s higher 7.9% figure triggered great concern, particularly from readers who did not realize that this was an extreme and unlikely limit.
Environmental Defense Fund done by the Rhodium Group
22
estimates the world-wide
leakage to be about 3%. The effects of leakages are easily read off Table 3; the advantage
With a 100-year GWP of 34, many thought
remains strong for natural gas.
(incorrectly) that a 3% leak would negate all advantage over coal. However even the 7.9%
A similar conclusion was reached in a recent
leakage number is not disastrous when we take
paper by J. Peischi et al.23 They determined from
into account the efficiency of natural gas
airplane measurements that for the predominant
generators. For a 20-year average, 7.9% leakage
shale gas sites in the US, the fugitive methane
leads to a natural gas advantage of 0.78, that is,
leakage varies from a low of 0.18% to a high of
coal is better by a 22%. That’s not good, but it is
2.8%. The low levels that can be achieved by
not catastrophic. For the 100-year average at
following industry best practice are illustrated by
7.9% leakage, natural gas is still 40% better than
their measured leakage above the Marcellus:
coal. And at the 100-year point, the leakage is
from a low of 0.18 to a high of only 0.41% for that
virtually irrelevant – natural gas is advantageous
vast and highly fracked region (although this low
even if over half leaks.
number may have been achieved in part by other effects, such as fewer liquid unloadings in the dry gas found in this formation). This number
19
20
21
Brandt, A.R., Heath, G.A., Kort, E.A., O’Sullivan, F., Pétron, G., Jordaan, S.M., Tans, P., Wilcox, J., Gopstein, A.M., Arent, D., Wofsy, S., Brown, N.J., Bradley, R., Stucky, G.D., Eardley, D., Harriss, R., 2014. Methane Leaks from North American Natural Gas Systems. Science 343, 733-735. Z. Hausfather, Natural Gas Leakage in Brandt et al., Berkeley Earth memo, available at www.BerkeleyEarth.org/memos. Brandt concludes that overall US CH4 inventories from all sources are underestimated by 1.25x to 1.75x. The implied leakage rates depend on where the excess methane is coming from. You get 1.9% to 2.6% if you assume that the excess methane is distributed proportionately across known sources. There is evidence mentioned by Brandt that other sources
(e.g. lifestock) are also significantly underestimated. For details see Brandt and Hausfather. 22
K. Larsen, M. Delgado, P. Marsters, Untapped Potential, Reducing Global Methane Emissions from Oil and Natural Gas Systems. Available at: www.edf.org/sites/default/files/content/rhg_untappe dpotential_april2015.pdf
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Quantifying atmospheric methane emissions from the Haynesville, Fayetteville, and northeastern Marcellus shale gas production regions, J. Peischl, T. B. Ryerson, K. C. Aikin, J. A. de Gouw, J. B. Gilman, J. S. Holloway, B. M. Lerner, R. Nadkarni, J. A. Neuman, J. B. Nowak, M. Trainer, C. Warneke and D. D. Parrish, JGR Atmospheres, DOI: 10.1002/2014JD022697.
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does not include leakage from distribution and
goal was not to try to keep the model simple in
combustion, but it does illustrate that leakage at
order to maximise transparency and ease of use.
the wells can be kept very low.
Taking account of our different goal, we find no conflict of our results with those of Alvarez et al.
In terms of true legacy at 100 years (rather than average over the next 100 years), fugitive
Note that the 3.2% limit for acceptable leakage
methane is incapable of offering any threat
is the value for zero year lag. It is the period
whatsoever, because of its short 8.6 year half-life
immediately after the release, and when there is
in the atmosphere. Even with high leakage,
no
natural gas can be 3 times better than coal after
disappearance
100 years. If we were to compare future natural
atmosphere. For that reason, we think the
gas generators with high efficiency coal, this
emphasis on this number, not in the paper by
advantage drops to 2.
Alvarez et al., but in the focus put on it by policy
advantage
to of
methane methane
from
the
from
the
makers is misguided. In their paper, Alvarez et al. Hausfather has analysed a more complex situation, one in which the use of natural gas delays
the
advent
of
carbon-free
power
generation. If this happens, the 100-year benefit of natural gas is reduced, but in most cases there is still a benefit.24
7. COMPARISON WITH PRIOR RESULT OF ALVAREZ ET AL. The most widely quoted number for the acceptable limit for natural gas is that found in the publication by R. Alvarez et al. of 3.2%. This is
also give the leakages that would achieve equivalence for longer periods. For example, in their Fig. 2C, they show that a 7% pulse of leakage would achieve equivalence to coal in about 45 years. It is important to note that what they mean by this result is that the average over 45 years is equal to that of coal. After 45 years (over 5 half-lives) 97% of the leaked methane is gone. It is easy to misread the Alvarez et al. results to think that methane has a long legacy.
the number that has been used by policy makers
8. CHINA, INDIA, AND THE DEVELOPING WORLD
to determine acceptable leakage. It compares to
When considering energy policies in the US, the
the value of 3.8% in our Table 2, for zero year lag.
UK, or Europe, it is important to consider how
Our number is slightly higher than that of Alvarez
insignificant the West is in the future of global
et al. for several reasons. They assumed coal
warming. The future rise in global temperatures
efficiency at 39% (vs. our 33%). That difference is
will come primarily from China, India, and the rest
attributable to different goals; they wanted to
of the developing world. The developed world
compare to future coal plants, and we were
can hope to set an example that the developing
comparing to the ones that natural gas would
nations can then follow, but it needs to be an
replace. They assumed 50% efficiency for natural
example that they can afford.
gas, and we took a higher level of 54%, to see what is the best we could achieve if the global
As a specific example, suppose that the US were
warming considerations were taken to be
to replace half of its coal-powered stations
important for the design. They also used a more
immediately, today, with zero-carbon power
detailed model of coal, including methane
plants. About 20% of the total US energy use
leakage from its mining and other factors. Our
comes from coal; let’s assume that 30% of its
24
10
Z. Hausfather, Bounding the Climate Viability of Natural Gas as a Bridge Fuel, published in Energy
Policy, vol. 86, pp. doi:10.1016/j.enpol.2015.07.012.
286-294
(2015);
CO2 comes from this. A cut in half would amount
regulations that require industry best practice at
to a 15% cut in the CO2 from the US. Let’s also
all wells. Because of the short 8.6-year half-life of
assume that China reduces its emission growth
methane in the atmosphere, the legacy danger
to the promised level of 6% per year. Since
of fugitive methane is tiny. If methane leakage
China’s emissions are now double those of the
proves to only be a temporary phenomenon. If
US, China’s growth would negate the reduction
we continue to use natural gas and sustain a
in the US in 15 months.
high leakage rate over the full century, methane leakage has more of an impact, although still less
Thus even the unrealistic scenario of cutting US
than coal for any plausible leakage rate.
coal use by 50% would result in only a trivial delay in warming. If the goal is to prevent
It is always worthwhile to emphasise that
substantial additional global warming, the focus
reduction of greenhouse emissions in the US, the
must be on the expected rise in emissions from
UK, and Europe is a worthwhile goal, but it is the
the developing world. The West must help the
developing world that really counts. We need to
developing world avoid new coal use.
set an example that China and the rest of the developing world can afford to follow.
China and India have an additional reason to switch from coal to natural gas: the fact that particulate air pollution can be reduced by a factor of 400 by doing so.25 The poorer nations can’t afford to subsidise carbon-free energy, so in
general,
economic
concerns
must
be
foremost. In much of the developing world, coal provides the primary source of electric power, and to the extent that natural gas can replace it, both greenhouse gas and air pollution emissions can be substantially reduced.
9. DISCUSSION The benefits of natural gas for electricity production compared to that of coal are large, and the role it could play as a bridging fuel is significant. Our main concern is for the future, and that is why we assumed replacement of existing coal facilities with high efficiency combined cycle natural gas generators. Many currently existing natural gas plants don’t have the high efficiency we assumed, and that reduces their “advantage” factor. The threat of fugitive methane is low, and could be made even lower by addressing the small number of super emitters, primarily through 25
R. Muller and E. Muller, Why Every Serious Environmentalist Should Favour Fracking, Centre for
Policy Studies (London, 2013), ISBN 978-1-906996-802, available at www.BerkeleyEarth.org/papers.
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THE AUTHORS Elizabeth Muller is co-founder and Executive Director of Berkeley Earth, a non-profit research organization. She has authored numerous scientific and policy papers, and Op Eds in the New York Times and the San Francisco Chronicle, and made numerous TV and radio appearances. Previously she was a director at Gov3 (now CS Transform) and Executive Director of the Gov3 Foundation. From 2000 to 2005 she was a policy advisor at the OECD (Organization for Economic Cooperation and Development). She has advised governments in over 30 countries, in both the developed and developing world, and has extensive experience with stakeholder engagement and communications, particularly regarding technical issues. Richard A Muller has been Professor of Physics at the University of California, Berkeley since 1980. He is recognised as one of the world’s leading climate scientists and is the co-founder and scientific director of Berkeley Earth, a non-profit organization that reanalysed the historic temperature record and addressed key issues raised by climate sceptics. He is the author of Physics for Future Presidents and Energy for Future Presidents and six other books. He has founded two projects that led to Nobel Prizes and was named by Foreign Policy as one of its 2012 Top 100 Global Thinkers.
The aim of the Centre for Policy Studies is to develop and promote policies that provide freedom and encouragement for individuals to pursue the aspirations they have for themselves and their families, within the security and obligations of a stable and lawabiding nation. The views expressed in our publications are, however, the sole responsibility of the authors. Contributions are chosen for their value in informing public debate and should not be taken as representing a corporate view of the CPS or of its Directors. The CPS values its independence and does not carry on activities with the intention of affecting public support for any registered political party or for candidates at election, or to influence voters in a referendum.
ISBN 978-1-910627-17-4 Centre for Policy Studies, October 2015
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